1. FIELD OF THE INVENTION
The present invention relates to a ball grid array integrated circuit package.
2. DESCRIPTION OF RELATED ART
Integrated circuits are typically housed within a plastic or ceramic package. Some integrated circuit packages contain a plurality of pins that extended from the package and are soldered to corresponding plated through holes in a printed circuit board. The plated through holes reduce the amount of routing space within the circuit board. Integrated circuits are typically designed to provide more digital signal inputs and outputs (I/O), which require additional routing within the printed circuit board. More electrical routing traces typically required additional layers or thinner trace line widths. Both solutions increase the cost of manufacturing a printed circuit board.
There has been developed electronic package assemblies which mount the integrated circuit package to the surface of the printed circuit board. The integrated circuit package has a number of pins or solder pads that are soldered to corresponding surface pads of the printed circuit board. The integrated circuit package is typically soldered to the printed circuit board by solder paste that is screened onto the surface pads before the package is mounted to the board. Surface mounted integrated circuit packages are typically constructed from co-fired ceramic or molded plastic material.
U.S. Pat. No. 5,216,278 issued to Lin et al. discloses a surface mounted package commonly referred to as a ball grid array (BGA). BGA packages are typically constructed with a substrate that supports the integrated circuit and which is constructed with conventional printed circuit board processes. The surface of the substrate opposite from the integrated circuit contains a plurality of solder pads. After the integrated circuit is mounted and attached to the substrate (and typically enclosed with an encapsulant) a solder ball is placed onto each solder pad. The solder balls are then heated and reflowed to attach the balls to the pads. The solder balls are then subsequently soldered to a printed circuit board to attach the package to the board. A pattern of solder mask is typically placed onto the surface of the substrate to prevent undesirable cross-wicking of solder between solder pads. To allow the solder balls to become attached to the solder pads, the solder mask has a plurality of openings located concentrically with the pads. The solder mask openings have a diameter that is less than the diameter of the solder pads.
As shown in FIG. 1, existing BGA packages have vias 2 that interconnect the solder pads 4 with the opposite side of the substrate. The vias 2 are separated from the solder pads 4 in a "dogbone" configuration. The bottom surface of the package has a layer of solder mask 6. As discussed in the Lin patent, the vias are separated from the solder pads so that solder does not flow into the vias.
By separating the vias from the solder pads, the dogbone configuration creates a less than optimal trace pattern which occupies valuable routing space on the surface of the substrate. It would be desirable to have a BGA package substrate that provides more routing space than substrates of the prior art.
The temperature of the package is elevated when the solder balls are reflowed onto the solder pads and elevated again when the solder balls are attached to the printed circuit board. The reflow process can vaporize moisture trapped in the barrels of the vias. It is important to provide a package construction that allows the vaporized moisture to "outgas" from the vias. Trapped vaporized moisture may cause delamination and degradation of the package. Therefore it would be desirable to have a BGA package that provides optimum routing space and allows outgassing from the vias.